JP7831430B2 - Method for manufacturing pass line rolls and steel plates - Google Patents

Description

This invention relates to a pass line roll, which is disposed between multiple rolling mills in a rolling line having multiple rolling mills, and to a method for manufacturing steel sheets.

In steel sheet rolling, tandem rolling is performed by arranging multiple rolling mills in a tandem configuration along the rolling line and passing the steel sheet to be rolled through these mills. In tandem rolling, measuring and controlling the tension between the rolling mills is essential to roll the steel sheet to a predetermined thickness. Therefore, pass line rolls are installed between the rolling mills, and these pass line rolls are used to measure and control the tension in the rolling line. Specifically, the tension on the exit side of the rolling mills located upstream of the rolling line is measured and controlled relative to the pass line rolls. The pass line rolls include a tension meter roll for measuring the tension in the rolling line and a defloor used as an auxiliary roll for the tension meter roll.

In tandem rolling, slippage occurs between the steel sheet and the rolls due to the speed difference between the steel sheet being passed and the rolls on the rolling line (work rolls and pass line rolls in the rolling mill). In contrast, Patent Document 1 discloses a method to prevent slippage between the work rolls and the steel sheet by adjusting the surface roughness Ra of the work rolls.

Furthermore, slippage also occurs between the steel plate and the pass line roll. Because the pass line roll operates using a non-driven rotation system, its rotation stops when slippage occurs. Therefore, when slippage occurs between the steel plate and the pass line roll, scratch-like defects appear on the surface of the steel plate. These scratch-like defects are more likely to occur with thin steel plates, specifically those with a thickness of less than 0.6 mm.

Therefore, to prevent slippage between the steel sheet and the pass line rolls, a method is employed in which roughness is applied to the surface of the work rolls of a rolling mill located upstream of the pass line, and this roughness is transferred to the surface of the steel sheet through rolling by these work rolls. This method increases the frictional force (coefficient of friction) between the steel sheet and the pass line rolls as the sheet passes through the pass line rolls, thereby suppressing slippage. Furthermore, for thin materials where higher surface quality is required, to avoid slippage between the thin material and the pass line rolls, the work rolls are replaced with those that have been given surface roughness prior to rolling on the rolling line.

Japanese Patent Application Publication No. 08-24903

However, the method of increasing the coefficient of friction between the steel plate and the pass line roll by imparting roughness to the surface of the work roll and transferring that roughness to the surface of the steel plate has a significant impact on the surface quality of the steel plate, making its active application difficult. Furthermore, because the surface of the work roll undergoes remarkably rapid wear due to repeated rolling, suppressing slip by transferring the roughness of the work roll surface to the surface of the steel plate is practically difficult in operation.

This invention has been made in view of the above circumstances, and its objective is to provide a pass line roll that can avoid affecting the surface quality of steel sheets and suppress the occurrence of slippage between the pass line roll and the steel sheets, when the pass line roll is installed between multiple rolling mills.

[1] A pass line roll disposed between a plurality of rolling mills in a rolling line having a plurality of rolling mills, wherein the surface roughness Ra of the contact surface with the steel sheet passed through the rolling line is 1.0 μm or more and 3.0 μm or less.
[2] The pass line roll according to [1], having a hard metal plating layer on the contact surface.
[3] A method for manufacturing steel sheets, comprising passing a steel sheet through a rolling line having a plurality of rolling mills and rolling it, wherein the steel sheet is passed through a pass line roll disposed between the plurality of rolling mills and having a surface roughness Ra of 1.0 μm or more and 3.0 μm or less on the contact surface with the steel sheet.

According to the present invention, it is possible to avoid affecting the surface quality of steel sheets and suppress the occurrence of slippage between the steel sheets and pass line rolls arranged between multiple rolling mills.

This figure shows a schematic side view of an example of a rolling line. This figure shows the relationship between the surface roughness transferred to the steel plate and the work roll pressure extension. This figure shows the relationship between the surface roughness of a pass line roll and its service life. This figure shows the relationship between the rotation speed of the work rolls and the rotation speed of the pass line rolls when pass line rolls with a surface roughness Ra of 0.2 μm or less are applied to a rolling line. This figure shows the relationship between the rotation speed of the work roll and the rotation speed of the pass line roll when a pass line roll with a surface roughness Ra of 2.5 μm is applied to a rolling line.

The embodiments of the present invention will be described in detail below. Figure 1 shows a schematic side view of an example of a rolling line 1. The rolling line 1 may be a rolling facility capable of tandem rolling with multiple rolling mills. The rolling line 1 has, in order from the upstream side along the sheet passage direction T of the steel sheet S, an upstream rolling mill 10, a pass line roll 20, and a downstream rolling mill 30.

The upstream rolling mill 10 has a pair of upper and lower backup rolls 11 and a work roll 12, sandwiching the steel sheet S. The pass line roll 20 has a tension meter roll 21 and a deflorer 22. The tension meter roll 21 measures the tension of the steel sheet S in the rolling line 1. The deflorer 22 is used as an auxiliary roll for the tension meter roll. The upstream rolling mill 10 and the downstream rolling mill 30 roll the steel sheet S as it passes through the rolling line 1.

Here, we will explain the relationship between the surface roughness of the steel sheet and the work roll pressure extension when roughness is applied to the surface of the work roll and transferred to the surface of the steel sheet, thereby increasing the frictional force between the steel sheet and the pass line roll, using Figure 2. Figure 2 is a diagram showing the relationship between the transferred surface roughness on the steel sheet and the work roll pressure extension. The work roll pressure extension refers to the total length of the steel sheet rolled using the work roll.

As shown in Figure 2, when the work roll pressure extension is short, the surface roughness Ra of the steel plate is in the range of 0.4 to 0.6 μm. Therefore, it can be confirmed that when the work roll pressure extension is short, the occurrence of slip between the steel plate (with its transferred roughness) and the pass line roll is suppressed. In other words, it is thought that by maintaining a high surface roughness Ra of the steel plate, the frictional force (coefficient of friction) between the steel plate and the pass line roll increases, thereby suppressing slip.

On the other hand, as the work roll pressure extension increases, the surface roughness Ra of the steel sheet falls to a value of 0.4 μm or less, and it can be confirmed that slip is more likely to occur between the steel sheet, whose roughness has been transferred, and the pass line roll. This can be confirmed by the fact that slip occurs at the timing when the surface roughness Ra of the steel sheet decreases, as shown in Figure 2.

Therefore, in order to suppress slippage between the steel plate and the pass line roll, the frictional force (coefficient of friction) between the steel plate and the pass line roll is a crucial factor, and it is necessary to increase the frictional force (coefficient of friction).

Furthermore, the relationship between the surface roughness of the steel sheet and the work roll pressure extension shown in Figure 2 is based on the premise of transferring the surface roughness of the work roll to the surface of the steel sheet. Therefore, it has a significant impact on the surface quality of the steel sheet, making its active application difficult.

Therefore, in this invention, slippage between the pass line roll and the steel sheet passing through it is suppressed by imparting roughness to the surface of the pass line roll. Specifically, by setting the surface roughness Ra of the pass line roll to 1.0 μm or more, it is possible to suppress the occurrence of slippage. In rolling using the rolling line 1, a threshold of 1.0 μm is set for the surface roughness Ra of the pass line roll, and if the surface roughness Ra falls below 1.0 μm, the pass line roll is replaced.

In the rolling mill capable of tandem rolling (rolling line 1), a dull roll with a high surface roughness is used in the downstream rolling mill among the multiple rolling mills. The dull roll has a surface roughness Ra value exceeding 3.0 μm. Therefore, when manufacturing steel sheets in the rolling mill, the surface roughness of the steel sheets is controlled (adjusted) by using a dull roll with a final surface roughness Ra value exceeding 3.0 μm. Based on this, and from the perspective of the impact on the surface quality of the steel sheets, the upper limit of the surface roughness Ra of the pass line rolls will be set to 3.0 μm or less in order to minimize the influence of the dull rolls on the control of the steel sheet's surface roughness Ra.

Here, based on the surface roughness Ra range of the pass line roll 20 (1.0 μm to 3.0 μm), we investigated the progression of the surface roughness Ra when a pass line roll 20 with a surface roughness Ra value of 2.5 ± 0.5 μm was applied to the rolling line 1.

The progression of the surface roughness Ra of the pass line roll 20 will be explained using Figure 3. Figure 3 shows the relationship between the surface roughness of the pass line roll 20 and its service life. As shown in Figure 3, at the point when the pass line roll 20 with a surface roughness Ra value of 2.5 ± 0.5 μm was applied to the rolling line 1 (left end of the graph), it can be confirmed that the surface roughness Ra value was 2.0 μm or higher. Furthermore, it was confirmed that the surface roughness Ra value could be maintained at 1.0 μm or higher even after one year of use.

The surface roughness of the pass line roll 20 may be imparted (surface processed) by methods such as shot blasting, sandblasting, or grid blasting. Alternatively, compared to shot blasting, dimple processing, which reduces localized pressure concentration by creating spherical surface roughness, may also be used to impart surface roughness to the pass line roll 20.

It is preferable to apply a hard metal plating layer to the contact surface of the pass line roll 20 after surface roughening (surface processing). Having a hard metal plating layer on the contact surface allows the surface roughness achieved through surface processing to be maintained over a long period. In this case, the hard metal plating layer may be formed, for example, starting from the side closest to the surface of the pass line roll 20, with the first layer formed from a Ni-based self-fluxing alloy containing Ni or Cr with a hardness of approximately HV650, and the second layer formed by WC cermet processing with a hardness of approximately HV1000. Even when the hard metal plating layer is formed as a single layer by WC cermet processing, it is effective in maintaining surface roughness. However, due to the large difference in hardness between the pass line roll 20 and the hard metal plating layer, delamination between the pass line roll 20 and the hard metal plating layer is likely to occur. Therefore, it is preferable to form a layer with a lower hardness than the hard metal plating layer between the hard metal plating layer and the pass line roll 20.

Furthermore, the steel sheet manufacturing method according to the present invention involves rolling a steel sheet S through a rolling line 1 having multiple rolling mills (upstream rolling mill 10 and downstream rolling mill 30). Specifically, the steel sheet S is passed through a pass line roll 20, which is positioned between the multiple rolling mills (upstream rolling mill 10 and downstream rolling mill 30) and has a surface roughness Ra of 1.0 μm or more and 3.0 μm or less at the contact surface with the steel sheet S.

Next, we will explain the results of applying the pass line roll according to the present invention to a rolling line and performing rolling processing by passing a steel sheet through the rolling line. The steel sheet passing through the rolling mill passes at a speed equivalent to the rotational speed of the work rolls in the rolling mill. Since the pass line roll, located downstream of the rolling mill, is a non-driven type, it rotates at a speed equivalent to the steel sheet's passing speed due to contact with the steel sheet.

Based on these speed-related relationships, this embodiment evaluated the relationship between the rotational speed of the work rolls and the rotational speed of the pass line rolls in the rolling mill. Specifically, when the rotational speed of the work rolls and the rotational speed of the pass line rolls were equivalent, the pass line rolls were evaluated as "no slip." When the rotational speed of the work rolls was faster than the rotational speed of the pass line rolls, the pass line rolls were evaluated as "slip present."

Figure 4 shows the results of investigating the relationship between the rotation speed of the work rolls and the rotation speed of the pass line rolls when a pass line roll with a surface roughness Ra of 0.2 μm was applied to the rolling line. Figure 5 shows the results of investigating the relationship between the rotation speed of the work rolls and the rotation speed of the pass line rolls when a pass line roll with a surface roughness Ra of 2.5 μm was applied to the rolling line. In both cases, surface roughness was imparted by the shot blasting method. The steel sheets used in the rolling line had a tensile strength of 270 MPa and an estimated surface roughness Ra of approximately 0.3 μm. The rolling speed in the rolling line was set to approximately 1000 mpm. No work rolls were replaced before rolling. The surface roughness Ra of the pass line rolls described above was measured based on "JIS B 0601:2013 Surface Roughness," with an evaluation length of 4 mm and a cutoff of 0.8 mm.

As shown in Figure 4, when a pass line roll with a surface roughness Ra of 0.2 μm was applied to the rolling line, a large speed difference was observed between the work roll rotation speed and the pass line roll rotation speed, confirming that slip occurred between the pass line roll and the steel sheet.

On the other hand, when a pass line roll with a surface roughness Ra of 2.5 μm was applied to the rolling line, as shown in Figure 5, the rotational speed of the work roll and the rotational speed of the pass line roll always matched, confirming that no slip occurred between the pass line roll and the steel sheet.

As described above, by imparting surface roughness to the contact surface between the steel sheet and the pass line roll, and increasing the frictional force (coefficient of friction) between the steel sheet and the pass line roll, slippage between the steel sheet and the pass line roll was suppressed regardless of the surface roughness of the steel sheet. Furthermore, the need to increase the frictional force (coefficient of friction) between the steel sheet and the pass line roll by imparting roughness to the surface of the work roll and transferring that roughness to the surface of the steel sheet was eliminated, thus avoiding any impact on the surface quality of the steel sheet. Moreover, based on the suppression of slippage, it is possible to improve the surface quality of the steel sheet and the yield in steel sheet manufacturing.

1 Rolling line 10 Upstream rolling mill 11 Backup roll 12 Work roll 20 Pass line roll 21 Tension meter roll 22 Defroll 30 Downstream rolling mill S Steel plate T Direction of plate passage

Claims (2)

A pass line roll disposed between a plurality of rolling mills in a rolling line having a plurality of rolling mills, wherein the surface roughness Ra of the contact surface with the steel sheet passed through the rolling line is 1.0 μm or more and 3.0 μm or less, and the contact surface has a hard metal plating layer consisting of a Ni-based self-fluxing alloy layer and a layer formed by WC cermet processing, in order from the side closest to the surface of the pass line roll . A method for manufacturing steel sheets, comprising passing a steel sheet through a rolling line having multiple rolling mills, wherein the steel sheet is passed through a pass line roll having a hard metal plating layer on its contact surface, which is disposed between the multiple rolling mills and has a surface roughness Ra of 1.0 μm or more and 3.0 μm or less on the contact surface with the steel sheet, and which has a layer of Ni-based self-fluxing alloy and a layer formed by WC cermet processing in order from the side closest to the surface of the pass line roll, in which case the steel sheet is passed.